Lasers have found widespread industrial applications in areas including but not limited to welding, cladding, transformation hardening, annealment, and the like. Laser material processing takes advantage of the ease at which the laser light can be optically configured, controlled and directed onto a material's surface.
Each laser process can be characterized by the power density used to perform the desired material modification. In laser welding a power density typically between 3.times.10.sup.6 W/in.sup.2 and 2.times.10.sup.7 W/in.sup.2 is used. When this power density is brought to a metal's surface a hole is formed, filled with metal vapor and plasma. This is known as a keyhole in this high energy welding field. With relative motion the keyhole is moved down the weld seam. With a given power density the rate of relative motion controls the depth of the keyhole and thus the depth of the weld.
Laser transformation hardening of steels generally requires a power density between 3 KW/in.sup.2 and 32 KW/in.sup.2. Typically this power density is transverse across the section to be hardened. A combination of travel speed and laser spot size produce a dwell time to which the steel is brought to a temperature above its phase transformation temperature to a specific depth without melting the surface. The relatively low temperature of the bulk material provides the rapid quench required to produce a case of high hardness. In a few instances there is insufficient bulk to meet the quench rate requirements of the material so a coolant must be used. An unmolten metal is a good reflector of CO.sub.2 laser light so a coating is used to enhance the energy absorbtion.
Annealment with lasers requires a lower power density of about 3 KW/in.sup.2 to 10 KW/in.sup.2 and a coating is used for good power absorption. Benefits include high speed, selected areas of a workpiece can be annealed and low distortion is produced.
In a production environment it is possible for materials and part geometries to fall out of specification and machine tool components to deteriorate in performance or fail in use. Any of these can cause the process to go out of control resulting in a defective part. Statistical sampling of the processed parts is used to minimize the chance that a defective part continue through the system. To drive this chance to zero it becomes quite costly. It is of economic interest to develop systems to monitor processes on a part by part basis.
A real-time AE technique has been used to monitor laser beam welds, Jon, M. C. Welding Journal, 43, September 1985. This technique used a non-contacting sensor. A piezoelectric sensor was placed above the workpiece and monitored the pressure generated by the vaporization produced during laser welding. However, this AE method lacked sensitivity to noise immunity and output signal analysis and was generally limited in its practical applications.
Other methods and apparatus have been utilized in combination with non-laser welding operations for a variety of applications.
U.S. Pat. No. 4,532,404 noted that heat generated from a molten weld generated by an arc electrode was propagated as thermal waves through the bodies of respective metal pieces to be welded. Heat was generated in a radial manner and defined isothermal lines having progressively lowered values of temperature as the distance from the heating source increased. A pyrometer collected infrared rays emanating from the heated surfaces to define a temperature profile distribution at a time which reflected the real thermal dissipation condition prevailing ahead of the weld melt zone. A real time control system was used to adapt the operating characteristics of the welding electrode to environmental temperature variations or changes. The vertical position of the electrode was controlled in accordance with the monitored signals to compensate for vertical alignment. Other correction action such as displacement of the electrode over a colder edge or a tilting of the electrode tips could be effected.
U.S. Pat. No. 4,477,712 disclosed a method for seam tracking in a moving arc torch welding operation. The level of infrared radiation in the infrared band having wavelengths greater than 3 microns was determined for at least two points which were equidistant from the welding seam and positioned on opposite sides of the seam ahead of the torch welding direction. Signal information which was indicative of the temperature imbalance across the unjoined seam was generated and used to control the position of the arc torch.
U.S. Pat. No. 4,214,164 disclosed a method and control system for automatically operating a spot welder. Infrared radiation produced during the welding procedure was detected. The actual temperature of the weld was not itself measured but the infrared radiation was proportional to the temperature of the weld. A thermal signal was produced and compared to a value stored in a point set memory. The amount of electrical energy supplied to the spot welder was varied in time and intensity.
U.S. Pat. No. 4,484,059 disclosed an infrared sensor for an electric arc welder. An infrared detector received infrared radiation produced from the welding operation. A filter permitted passage of only infrared radiation having wavelengths greater than about 3 microns. The detector was utilized to obtain weld pool information by detecting infrared radiation emitted from the weld pool having wavelengths greater than 3 microns.
The methods and apparatus disclosed above are concerned only with welding operations. They fail to provide an apparatus or method useful for monitoring a laser process at a location behind of the laser process to automatically monitor the process. Such methods and apparatus have either utilized an AE sensor for laser welds or an infrared radiation sensor for non-laser applications. It would be an advancement in the art to provide a method and apparatus for monitoring a laser process by detecting infrared radiation at a point behind the laser process point. It would be a further advancement to provide a method and apparatus for monitoring laser processes whereby detected infrared radiation is compared to a predetermined signature range and if the detected infrared radiation falls outside the window a laser process station is signalled.